1. Field of the Invention
The invention concerns the exchange of encoded messages between stations and particularly, but not exclusively, between control units of a time-division exchange.
2. Description of the Prior Art
Systems are already known for exchanging encoded messages between stations operating in time-sharing mode. The stations may comprise computers or data receiving and sending devices or, more generally, data processing devices. In such message exchange systems, a message with priority higher than that of other messages takes precedence over such messages, requiring costly and complex switching arrangements.
One system for exchanging encoded messages between stations is disclosed in French Pat. No. 2 455 838. In a system of this kind each station comprises a connection module connected to a signal bus and to a data bus, which are serial buses synchronized by a timebase common to all the stations, a time interval comprising consecutive even-numbered and odd-numbered time slots being allocated in each signaling frame to each connection module. The signal and data buses operate at 1.28 Mbit/s, the data is exchanged in 32 time slots of a 125 microsecond frame and each time slot is subdivided into five intervals. The message length is fixed and limited to 157 bits. In this system the maximum number of stations interconnected is 15 and the priority of the stations on the data bus is fixed, station number 1 having the maximum priority, which means that at the end of message exchange from a sending station n, at the start of the signal bus frame station 1 is processed first, then station 2 if station 1 is not requesting, and so on until the first station in the requesting condition on the signal bus.
Thus in an exchange system of this kind the number of stations is limited, the messages have a fixed format and a limited length, and the priority of station access to the data bus is fixed.
As a result, an exchange system of this kind features constraints which limit or rule out its utilization.
An object of the invention is therefore a system for exchanging encoded messages between stations which does not have the disadvantages of the known exchange system.